dt-bindings: clock: st: flexgen: add new introduced compatible

New compatible are added, supporting various kind of flexgen in
STiH407, STiH410 and STiH418

Signed-off-by: Alain Volmat <avolmat@me.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210331201632.24530-4-avolmat@me.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>